In recent years thermal imaging technology has developed a capability of providing images of television quality or better for various applications, such as aerial terrain mapping, target determination and acquisition, surveillance, electrical fault location, medical imaging, and irrigation control.
One particularly useful technique for thermal imaging is known as "cool IR". This technique has the advantage of being able to carry out imaging over great distances, in total darkness, on camouflaged objects and through cloud cover. Cool IR systems require an IR detector to be cooled to the temperature of liquid air, about 77 K, for efficient operation.
Various types of cryogenic refrigerators are known for cool IR applications. These include liquid nitrogen cryostats, Joule-Thomson coolers and closed cycle cryocoolers. For certain applications, closed cycle cryocoolers are preferred.
There exist a variety of configurations of closed cycle cryocoolers. These include Stirling, Vuilleumier (VM) and Gifford-McMahon (GM) cryocoolers. A preferred configuration is the integral type.
A basic integral Stirling cryocooler comprises a compressor section and an expander-displacer section combined in one integrated package. Reciprocating elements of both the expander-displacer and the compressor are mechanically driven via a common crankshaft. The integral configuration guarantees a prescribed displacer stroke and displacer/compressor phase relationship, but it involves a disadvantage in that the vibration output of the compressor is transmitted to the cooled device due to the close proximity of the components.
A further disadvantage in integral Stirling crycoolers lies in their compressor seals. Various types of dynamic compressor seals are employed, including clearance seals. These tend to wear over time, releasing particulate matter into the system; this interferes with the operation of the Stirling regenerator.
Additional contamination of the regenerator is caused by lubrication materials and other materials associated with parts of the drive motor which are generally located in fluid communication with the regenerator.